Centrifugal compressors supply oil-free compressed gas in a variety of industrial applications. A common application of a centrifugal compressor is in plant air systems, to supply a motive force for valve actuators and pneumatic cylinders used in robotic applications, as one example. Centrifugal compressors typically feature an impeller mounted in a closely-conforming impeller chamber. The chamber features an axial inlet port to allow fluid entry toward the center of the impeller. Fluid is drawn into the impeller due to its rotation at speeds that can exceed 75,000 revolutions per minute (RPM). The rotation of the impeller propels the fluid through an annular diffuser passageway and into a surrounding volute. The energy imparted into the fluid by the impeller's rotation increases the fluid's velocity and, consequently, pressure as the fluid passes the diffuser passageway into the scroll or volute. The diffuser passage way has inside and outside radial dimensions for each circumferential station of the impeller chamber and scroll. By definition, the inside radius of the diffuser section corresponds to the distance to the diffuser throat or the location at which the annular port or passageway has the smallest axial width for the given station, the diffuser section extending outwardly for the remainder of the annular passageway.
Traditionally, centrifugal compressors have featured a bolt on scroll/volute cover, which encompassed portions of the impeller chamber, the diffuser passageway and the volute-outlet passageway. U.S. Pat. No. 4,181,466 is illustrative of a bolt-on component featuring a fluid entry 51 and a volute 50 that is secured to the bearing housing 15 by a V-clamp 49. A difficulty with the bolt-on scroll/volute cover incorporating the volute is the effective control of tip clearance between the impeller and the inlet passageway and the clearance between the impeller and the volute outlet. Due to the bolt-on construction previously employed, machining costs and assembly costs affected the finished cost of the product. The assembly of a plurality of components required the use of greater clearances around the impeller, which sacrificed compressor efficiency. This, in turn, required larger drivers and higher operating costs for electric power. Since each assembled component has a manufacturing tolerance, the final clearance near the impeller has to be sufficiently large to accommodate a situation where all the tolerances in the individual components of the assembly turned out within specification but all dimensions on the individual components were off from the ideal dimension and on the same side of the tolerance allowed.
Another problem with bolt-on volutes, i.e., 24 and 26, is the extra space and mass taken up by that type of assembly. Such space could become important in situations where ease of installation and maintenance is important to serviceability. For example, as will be explained below, use of bolt-on volutes (such as 24 and 26) hinders access to the driver shaft for an oil pump to be directly driven. The extra housing thickness for each stage in a multi-stage skid could preclude a direct drive on the oil pump and may necessitate a separate electrical drive for the oil pump. This would be undesirable in the event of an electrical failure. In an electrical failure, the impeller bearings need lubrication, as the impeller slows from its operating speed of 75,000 RPM or more. Bearing failure could result with an electrically driven oil pump if it stopped delivering oil too abruptly on power failure. A power takeoff from the main drive shaft, which could involve gears or belts, adds to the complication of packaged systems and tends to complicate access when maintenance is required.
One issue that remains unresolved by the integral casting of the volute as part of the gearbox is what can be done if the end user needs a capacity change that involves a speed change to one or more stages in a compressor assembly. Normally, such a speed change involves a gear ratio change. Typically, the end user prefers to simply change a pinion 34 shown in FIG. 2 while retaining the much larger bull gear 32. The reasons for this preference are cost and speed of getting the replacement parts. It is far easier for the original equipment manufacturer to stock a variety of pinions than to have a lot of cash tied up in very large parts such as different bull gears 32. However, using a different sized pinion with the same bull gear changes the center to center distance between them and the scroll is integrally cast to a fixed center to center distance.